This invention relates to methods of simulating events taking place in a plurality of press machines, each press machine having at least one die opening and matching upper and lower punches.
There exists a multitude of press machines that are presently in widespread use for compacting powdered materials into solid or semisolid form by exerting force on at least one set of two opposing punches or pistons entering once or twice a plurality of dies or pressing matrices containing the material to be compacted (e.g. U.S. Pat. Nos. 4,408,975 to Hack, U.S. Pat. No. 4,569,650 to Krxc3xa4mer, U.S. Pat. No. 4,680,158 to Hinzpefer et al., U.S. Pat. No. 4,880,373 to Balog et al., U.S. Pat. No. 5,017,122 to Staniforth, U.S. Pat. No. 5,116,214 to Korsch et al., U.S. Pat. No. 5,148,740 to Arndt et al., U.S. Pat. No. 5,202,067 to Solazzi et al., U.S. Pat. No. 5,211,964 to Prytherch et al., U.S. Pat. No. 5,462,427 to Krxc3xa4mer and U.S. Pat. No. 5,607,704 to Schlierenkxc3xa4mper et al.). A number of inventions relate to press machine instrumentation (e.g. U.S. Pat. No. 3,255,716 to Knoechel et al., U.S. Pat. Nos. 4,016,744, 4,030,868 and U.S. Pat. No. 4,099,239 to Williams, U.S. Pat. No. 4,100,598 to Stiel et al., U.S. Pat. No. 5,229,044 to Shimada et al.) and control (e.g. U.S. Pat. No. 3,734,663 to Holm, U.S. Pat. No. 4,121,289 to Stiel, U.S. Pat. No. 4,570,229 to Breen et al., U.S. Pat. No. 4,817,006 to Lewis, U.S. Pat. No. 5,288,440 to Katagiri et al., U.S. Pat. No. 5,491,647 to O""Brien et al.).
The examples of applications using press machines include pharmaceutical tablets and caplets, coal briquettes, ammunition, nuclear pellets, metal and plastic machine parts, ceramic isolators, catalysts or ferments, briquettes for X-ray spectrochemical analysis, grain pellets, coins, and so on.
In a compaction process, the mechanical and other properties of the compound are influenced primarily by powder composition, as well as by speed, movement profile and the force of punches that are in contact with the powder under compression. In a typical production environment, compressed products are usually made in large quantities at fast speeds. During a product development stage and for process troubleshooting, smaller quantities of the powder are often available while the press machines may be much slower and, in general, quite different from those used in production. For a product and process optimization, therefore, it is desirable to be able to reproduce typical production conditions to avoid problems in the scale-up of processing factors.
In the prior art, compaction simulators based on hydraulic actuators are used for the purpose of mimicking the compaction profile of different press machines. Typically, a pump pushes pressurized oil to the cylinder units that, in turn, move the punch holders with the help of valves and hydraulic tanks. The movement of the two punches entering the die cavity with the material to be compacted can be controlled by the actuators in order to follow any prescribed path. The path is specified in the form of a geometrical function (such as a sinusoid or a tooth-saw waveform) or may have any arbitrary form as recorded during a compaction event on another press machine with the aim of mimicking this event on the simulator. The recording from another press machine may contain either the punch displacement path or the force change profile. The desired path, whether theoretical or empirical, is further digitized by a computer, and a series of discrete commands are then given to the hydraulic actuators that are diligently reproducing either the movement of the punches or the force curve (see, e.g. U.S. Pat. No. 5,517,871 to Pento).
There are several problems with the existing compaction simulators that render them practically useless for process scale-up:
The hydraulic actuators can follow any prescribed path but theoretical paths such as a sinusoid are not representing the punch movements in the production press. Fixed geometry of the functions used to produce theoretical waveforms do not take into account the compressibility of the powders under compression (for force curve simulation) or the mechanical deformation of the punches and press assembly (for punch displacement simulation).
The empirical waveform that can be obtained from a production press depends on the brand and model of the press, the shape and size of the tooling, the production rate and the viscoelastic stress/strain behavior of the powder being compressed. Since the composition of optimized powder is unknown during the development stage, the present art solution is to use powder xe2x80x9csimilarxe2x80x9d to the one being developed even though the degree of similarity can never be quantified or made sufficient for quantitative analysis of the compatibility. In addition, the multitude of possible values of such factors as tooling, press speed and geometry make this empirical approach to compaction simulation highly impractical.
In the currently available compaction simulators, the motion of punches is controlled by hydraulic actuators that periodically compare the current position or the force of the punches with the digitized prescription. Such comparison and the subsequent correction can not be made with sufficient frequency to assure smooth trajectory without jerking or tooth-saw like movement, even with the fastest reported data acquisition and control rate of 5 kHz per channel.
In the currently available compaction simulators, there is a choice of simulating either the motion of the punches (punch penetration curves) or the force/time path (compression profile). It is impossible to mimic both.
As a result, there is a wide discrepancy between the resulting properties of compounds obtained from different simulators following the same prescribed path for the same compound. The reported difference of 10 to 16 percent have been attributed to elastic distortion and loading characteristics of the hydraulic systems.
The objective of this invention is to eliminate drawbacks of the currently available press simulators and the method they employ. The present invention provides new and improved methods of simulating any press machine and describes specifically a press simulating apparatus that represents but one embodiment of the methods described.
Specifically, the new methods include replication of the geometrical parameters of the press machines to be simulated, without any need for mimicking the punch path with the help of hydraulic mechanisms. The punch and die sets are selected to be identical with the target press machine to be simulated, while the geometry of compression and pre-compression path generating surfaces is maintained by means of interchanging wheels, so that the punches are forced to repeat the path of the target press due to mechanical dimensions of the tooling and the press parts involved.
In a preferred embodiment of the methods, the punches are moving in a linear motion with the help a belt. Since the arrangement is not rotary, the amount of powder required can be tightly controlled, and in fact, only one compound at a time can be produced and evaluated. The speed of the apparatus may be governed by means of a stepper or servomotor under a computer control that may match the desired speed of a target press in terms of the linear velocity of the punches.
The ejection of the compound from the die can follow the pattern of the target press by means of interchangeable eject cams that will repeat geometry of the cams on the presses to be simulated.
The punch displacement, as well as the force of pre-compression, compaction, and ejection can be measured by means of appropriate sensors known in the art.
The apparatus may be also equipped with a device for measuring the mechanical properties of each compound as it comes out of the die. In the proposed embodiment of the apparatus for pharmaceutical applications, each tablet after ejection can be positioned in a tablet tester for measurement of weight, thickness, diameter, or hardness. Immediate correlation between compression force or speed and the tablet properties can be established and displayed on the computer screen.
Thus the product and process can be optimized on the proposed apparatus using the proposed methods of simulating any press without a need for prescribing a specific digitized punch path or involving sophisticated and expensive hydraulic mechanisms. No measurement of punch displacement or forces are required albeit beneficial for quality control and experimental design.